Light modulation system including a superconductive plate assembly for use in a data transmission scheme and method

ABSTRACT

A method and apparatus for modulating light, wherein a light source provides light of a certain wavelength to be modulated by a layer of superconducting material which forms part of a specifically configured plate assembly. The superconducting layer is placed in the optical path of the light source. Further the superconducting layer is switched between a partially transparent non-superconducting state and a substantially non-transparent superconducting state by a modulation circuit. The resulting optical pulses transmitted through the superconducting layer are converted from the original wavelength to a lower wavelength by a frequency converting device.

BACKGROUND OF THE INVENTION

The present invention relates generally to fiber optic communication,and more particularly to modulation of light for use with fiber opticsin fiber optic communication schemes.

With the advent of dispersion compensating fibers, erbium doped fiberamplifiers, high speed amorphous silicon detectors, and all opticaldemultiplexing, fiber optic transmission speed is limited principally bythe modulation speed of the optical transmitter.

High speed modulators have been invented that take advantage of theproperties of superconducting materials. Superconducting materials arein there "superconducting" state if the current density in the material,and the temperature of the material, and the magnetic field around thematerial are all below certain critical values. The critical currentdensity (J_(c)), critical temperature (T_(c)), and the critical magneticfield (H_(c)) are all dependent on the chemical composition of thematerial and on the presence or absence of defects and impurities. Ifany of these quantities rise above the critical values the materialleaves its superconducting state and enters its "normal" state. Thematerial has properties similar to a semiconductor in it's normal stateand is characterized by a normal-state resistivity. The superconductingstate has many of the properties of a theoretically perfect conductor.The electrical resistance is zero and electromagnetic fields arereflected by it. Thus in the superconducting state a superconductingthin film acts like a mirror with 100% reflectivity 1, 2!. In the normalstate light is partially transmitted 3!. The bracketed end notereference numbers 1, 2! and all other such end notes and referencenumbers cited herein appear at the end of this specification along withthe reference notes themselves.

U.S. Pat. No. 5,210,637 which is incorporated herein by reference issuedMay 11, 1993 to Puzey for "High Speed Light Modulation" discloses adevice for the high speed modulation of light wherein a layer ofsuperconducting film is used to modulate the light. U.S. Pat. No.5,036,042 which is incorporated herein by reference issued Jul. 30, 1991to Hed for "Switchable Superconducting Mirrors" and discloses a devicethat can be used for the high speed modulation of light.

FIG. 1 herein illustrates one embodiment of U.S. Pat. No. 5,210,637 asindicated by reference numeral 10. A DC power supply 15 is connected toa light source 13 to provide constant light output. A superconductingfilm 14 is placed in the path of the optical output and its reflectivityis altered by a modulating circuit 16 which switches the film betweenits superconductivity and non-superconductive states, as described Inthe Puzey patent. The altered reflectivity results in optical pulses 20which are carried away by an optical fiber 25. The superconducting filmis kept cool by placing it in a dewar 22 which is cooled by means of arefrigerating device 26.

A key drawback of device 10 and the corresponding device in the HedPatent is that they are both limited to creating optical pulses in thefar infrared range (approximate wavelength of 14 microns). This isbecause at higher frequencies the photon energy of the light is highenough to break the binding energy of the Cooper electron pairsresponsible for the phenomena of superconductivity. In order for thedevice to work properly the photon energy of the light must be less thanthe binding energy (or energy gap) of the cooper pairs. This relation isgiven by the formula below:

    hv<2.increment.                                            {1}

Where h is Planck's constant, v is the frequency of light, and 2Δ is theenergy gap of the superconductor. The energy gap of the superconductorcan be found from Mattis-Bardeen 4!.

    2.increment.=8kT                                           {2}

Where k is Boftzman's constant, and T is the critical temperature of thesuperconducting material. High critical temperature Thallium compoundshave critical temperatures around 128 Kelvin. Plugging this intoequations {1} and {2}, the operation of the device is limited to lightwith a wavelength around 14 microns.

The attenuation of light in silica glass fiber (the most common materialfor long haul fibers) can be calculated from the formula below.

    α=Ae.sup.-/λ +B/λ.sup.4                {3}

Where α is the attenuation; A, a, and B are constants that are materialdependent. λ is the wavelength. The attenuation of 14 micron light insilica glass fiber is approximately 7.32×10¹⁰ dB per km using formula{3} and data from reference 5!. Therefor, applicant has found that theattenuation of 14 micron light in glass fiber is to high to be usefulfor telecommunication. Modern telecommunication systems are optimizedfor wavelengths around 1.3 or 1.55microns and have attenuation around0.15 dB per km. Unfortunately, light at these wavelengths (i.e. at thesehigher photon energies) are not compatible with the devices described inthe Puzey and Hed Patents. The present invention to be describedhereinafter provides a solution to this problem which has remainedunsolved since as long ago as December 1988, the filing date of the '042Hed patent.

SUMMARY OF THE INVENTION

The present invention provides an apparatus and method for themodulation of light. A layer of superconducting material is placed inthe optical path of a light source. The light source emits light with awavelength which in a preferred embodiment is long enough that theenergy of the individual photons is less than the superconducting gap ofthe superconductor. At least a portion of the superconducting layer isthen switched between a substantially non-transparent superconductingstate and a partially transparent non-superconducting state bypredetermined means. The optical pulses transmitted through the portionof the superconducting layer are then converted from the originalwavelength to a different wavelength, a shorter wavelength in thepreferred embodiment, by a frequency converting device. The shorterwavelength in the preferred embodiment has been chosen to allow anoptical fiber to efficiently carry the optical pulses withoutsignificant attenuation or dispersion.

In a specific embodiment of the present invention, the predeterminedswitching means is designed for switching a particular portion of thesuperconducting layer between its superconducting andnon-superconducting states using a modulating circuit thatintermittently raises the current density in the particular portionabove the critical current density of the superconducting material, orraises the magnetic field of the portion above the critical field of thesuperconducting material, or raises the temperature of the portion abovethe critical temperature of the superconducting material. The frequencyconverting device can be a parametric amplifier, parametric oscillator,Nth harmonic generator, four wave mixer, frequency upconverter, or anyother frequency converting device. Means for keeping the superconductinglayer below its critical temperature can be provided by placing thedevice in a dewar where at least a portion of the dewar is transparentto the longer wavelength and the dewar is actively cooled by a cryogenicrefrigerator.

The present invention may also include certain other features describedbelow. The present invention can include an optical fiber, opticallycoupled to the frequency converting device to conduct the optical pulsesaway from the device, for example to a receiver, optical demultiplexer,or other useful device. The present invention can include a number offiber optic links used to provide input data for the modulating circuit.This is useful because glass optical fibers conduct less heat than metalelectrical wires. Alternatively, free space optical links may be used toprovide data to the modulating circuit, eliminating a heat conductingpath into the dewar all together.

It is an object of the present invention to provide an improved lightmodulation device which increases the range of wavelengths which can bemodulated at an increased rate.

It is another object of the present invention to increase the rate atwhich data bits can be transmitted on a fiber optic link.

It is another object of the present invention to create high speedoptical pulses that are not limited to wavelengths with photon energieslower than the superconducting gap of the superconducting material.

It is an object of the present invention to reduce the amount of heatintroduced by incoming data sent to the modulating circuit.

The foregoing and other objects, features, and advantages of theinvention will be apparent from the following more particulardescription of the preferred embodiment of the invention as illustratedin the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic illustration of a prior art light modulationdevice employing superconducting material.

FIG. 2 is a diagrammatic illustration of a light modulating systemdesigned in accordance with the present invention and employing asuperconducting layer and a frequency converting device to modulatelight.

FIGS. 2A and 2B are diagrammatic illustrations of modified lightmodulating systems designed in accordance with the present invention;

FIGS. 3A-3H are detailed enlarged, not to scale diagrammaticillustrations of one embodiment of the superconducting layer shown inFIG. 2.

FIGS. 4A-4H are detailed enlarged, not to scale diagrammaticillustrations of an alternative embodiment of the superconducting layershown in FIG. 2.

FIG. 5 is a diagrammatic illustration of an alternative embodiment forthe superconducting layer.

FIGS. 6A and 6B are diagrammatic illustrations of still otheralternative embodiments of the superconducting layer.

DETAILED DESCRIPTION

Turning to the drawings, attention is immediately directed to FIG. 2,inasmuch as FIG. 1 was discussed above. FIG. 2 shows a light modulationsystem 10' designed in accordance with one embodiment of the presentinvention. This system includes most of the components described abovein conjunction with FIG. 1 (designated by the same reference numbersprimed) plus additional components to be described hereafter. Withparticular regard to the superconducting arrangement 14', as stated inthe Puzey patent electrical current of a certain minimum critical levelcan be used to switch superconducting arrangement 14' from itssuperconducting state to the normal state. Removing the electricalcurrent allows arrangement 14' to return to its superconducting state.When the current is below the critical current, the arrangement 14' isin the superconducting state and the optical output 20' is zero becauseof the 100% reflectance. When the current is above the critical currentthe material is in the normal state and the optical output 20' isnon-zero, that is some measurable level. Thus electrical current pulsescan be used to amplitude modulate the light from an optical source 13'.Note that this modulator has the ideal extinction ratio of zero.

Still referring to FIG. 2, in system 10', a plurality of fiber optictransmitters 31 and 32 are arranged to transmit light pulses in parallelto a plurality of receivers 35 and 36 through individual optical fibers33 and 34, see Van Zehgbroeck 6!. The optical transmitters 31 and 32 maybe current modulated laser diodes or LEDs. The receivers 35 and 36 maybe MSM (metal semiconductor metal) detectors. The electrical data fromthe signals are then read in parallel and serialized via a high speedshift register 16'. The shift register is made from Josephson Junctioncircuitry, see Martens et. al. 7!. A light source 13' is used togenerate light which is then amplitude modulated by arrangement ordevice 14' under the control of the serialized signals from the shiftregister 16'. Electrical energy is supplied to the light source 13' by apower supply 15'. The light source 13' may be an LED, laser, etc., as iscommonly known in the art. The modulating device 14' will be describedin more detail later.

The optical pulses 20' from the modulating device 14' enter a frequencyconverting device 41 which replicates the incoming pulses 20' in adifferent frequency of light 42 in accordance with one feature of thepresent invention. The frequency converting device 41 may be aparametric amplifier, parametric oscillator, Nth harmonic generator,four wave mixer, frequency upconverter, etc., some of the principles ofwhich are described in Yariv 8! and Saleh and Teich 9!. The frequencyconverting device 41 should be made from material that is transparent atthe wavelength of the modulated light 20' and the desired wavelength ofthe outgoing pulses 42, and has a high nonlinear conversion efficiency.The incoming pulses 20' are preferably on the order of 14 microns so asto be compatible with superconducting device 14'. The outgoing pulses 42are on the order of about 0.5 to 2 microns, preferably on the order of1.3 or 1.5 microns so as to be compatible with silica glass fibers.Suitable materials for the frequency converting device 41 are GaAs,ZnGeP₂, AgGaSe₂, Tl₃ AsSe₃, CdSe, AgGaS₂, Ag₃ AsS₃. These new pulses 42then enter an optical fiber 25' which carries the pulses and istypically made of silica glass. The pulses 44 exiting the fiber are thenreceived by an optical receiver 45. By way of illustration and notlimitation, the optical receiver 45 may be a high speed amorphoussilicon detector or an all optical demultiplexer and a plurality of lowspeed detectors. Such techniques have resulted in 100 Gb/s receivercapability, see Ronson et. al. 10!.

A dewar 22' is used in accordance with another feature of the inventionto thermally isolate the device 14' and shift register 16' from theoutside room temperature. The dewar 22' must be at least partiallytransparent to the optical energy 20' or have a window that issubstantially transparent to the optical energy 20'. A second windowcould be used to direct pulses to detectors 35 and 36 in lieu of opticalfibers 33 and 34 which extend into the dewar or the dewar could beentirely transparent. A cryogenic cooler 26' is used to keep thetemperature in the dewar below the critical temperature. The cryogeniccooler may be a sterling cycle refrigerator, Gifford-McMahonrefrigerator, tank of liquid nitrogen, etc.

FIGS. 3A-3H show an embodiment of the superconducting arrangement 14'.The modulator 14' is made by depositing a thin superconducting layer 50on a transparent or partially transparent substrate 49 such as siliconor diamond 11!. A film thickness of 480 angstroms transmits 6% of theincident light 3!. The critical current of the film depends on theproduct of the width, thickness, and critical current density. For a 480angstrom thick layer with a 100 micron bridge width and a criticalcurrent density of 10,000 Amps per square centimeter the criticalcurrent would be 480 micro Amps. This switch current would lead todissipative heating of about 96 nW for the same bridge 100 microns longwith a normal resistivity of 200 micro Ohms per centimeter. Theswitching speed of the film is limited by Abrikosov vortices nucleation.The modulation speed is given by 12!.

    t.sub.s-n =3t.sub.D (W/2Ω)(l.sub.c /l).sup.2,        {4}

where t_(s-n) is the superconducting to normal switching time, t_(D) isthe order parameter relaxation time, W is the bridge width, Ω is thedepairing ratio, l_(c) is the critical current, L is the switchingcurrent. Kozyrev estimates t_(s-n) to be on the order of a picosecondfor a 70 micron bridge. The FWHM spectral width of a transform limitedpulse is the reciprocal of the FWHM temporal width. For a pulse width ofabout a ps this gives a spectral width of a nanometer or so.

Still referring to FIG. 3A-3H, substrate 49 which is at least partiallytransparent to the optical pulses 20' is used to support the thin filmof superconducting material 50 which is H shaped so as to include legs50a and a bridge 50b. At the same time, segments 49a of substrate 49remain exposed in FIG. 3C. A dielectric layer 51 is used to electricallyisolate the superconducting layer 50 from a reflective layer 52 whichcovers segments 49a along with most of layer 51 as best seen in FIGS. 3Gand 3H. A conducting layer 53 which is elongated in configuration isplaced over and in direct contact with each leg 50A of material 50 andis used to provide electrical contact to the device 14'. The substrate49 may be made from MgO, silicon, diamond, etc. The superconductinglayer 50 may be made from niobium, yttrium, thallium, or mercury basedsuperconductors. Preferably the superconducting layer 50 is made from asuperconducting material with a high critical temperature, low normalresistivity, and low critical current density. The dielectric layer 51may be composed of silicon dioxide, spin on glass, polyimide, etc. Thereflective layer 52 is composed of a material that reflects opticalenergy 20' such as gold, copper, silver, metal, good conductors, etc.The reflective layer 52 prevents light from "leaking" around thesuperconducting bridge 50b. The conducting layers 53 are used to make agood electrical contact between the shift register 16' and thesuperconducting layer 50 via leads 54. The conducting layer 53 may bethe same material used for the reflecting layer 52. The conducting layer53 should have low electrical resistance and be substantiallyunreactive. Gold is a suitable material for both the reflecting layer 52and conducting layer 53.

Superconducting material only superconducts when the temperature of thematerial is below a certain temperature (called the criticaltemperature) and the magnetic field passing through the material isbelow a certain value (called the critical magnetic field) and theelectrical current density passing through the material is below acertain value (called the critical current density). Raising any ofthese three parameters above the critical value causes thesuperconductor to enter a non-superconducting state. In thesuperconducting state the superconducting material 50 is very conductiveand thus highly reflective. Electromagnetic energy is reflected in thisstate. In the non-superconducting state the superconducting material 50has properties similar to a semiconductor.

FIG. 2, which has been described, illustrates the use of the criticalcurrent density to control device 14'. FIGS. 2A and 2B illustrate theuse of a critical magnetic field and a critical temperature respectivelyto control the superconducting device 14'. Referring to FIG. 2A, element56 is a magnetic coil which is placed in proximity to device 14'. Thismagnetic coil 56 when provided with electrical current from shiftregister 16 via leads 54 raises the magnetic field above the criticalmagnetic field of material 50 causing it to enter a non-superconductingstate. Removal of the current from shift register 16 allows the material50 to re-enter a superconducting state. Similarly referring to FIG. 2B,element 57 is a resistor or other heating element placed in closeproximity to device 14'. This resistor 57 heats the material 50 aboveits critical temperature when provided with electrical current fromshift register 16 via leads 54. Removal of the current from shiftregister 16 allows the material 50 to re-enter its superconductingstate. In the non-superconducting state electromagnetic energy can betransmitted through the material 50. In the superconducting statematerial 50 substantially blocks transmission. Thus by placing thesuperconducting layer 50 in the path of the light from the light source13' the superconducting layer 50 can be used to control the transmissionor reflection of light under the influence of the electrical signalsfrom the shift register 16'. Recall that light is an electromagneticwave.

An alternative embodiment of the modulating device 14' is shown in FIGS.4A-4H. A substrate 49' that is substantially opaque to the opticalenergy 20' is used to support H shaped superconducting layer 50.Substrate 49' may be either highly reflective or absorptive. Thesubstrate 49' may be made from sapphire, lanthanum aluminate, galliumarsenide, or the like. The superconducting material may be anysuperconducting compound such as the niobium, yttrium, thallium ormercury based superconductors. An area 49c of the substrate under thebridge 50b of superconducting material 50 is at least partially removedto allow optical energy 20' to be transmitted through this area 49c. Thesubstrate material may be removed by ion milling, chemical etching,drilling, etc. A conducting layer 53 is used to provide a low resistanceelectrical contact between the legs 50a of superconducting layer 50 andthe shift register 16' as before.

Returning to FIG. 2, the addition of the frequency converting device 41allows the present system to overcome the problem of large attenuationdescribed above. A parametric amplifier as device 41 can be used to takethe high speed pulses 20' at 14 microns and convert them to high speedpulses 42 at a wavelength with lower attenuation and dispersioncharacteristics (such as 1.3 microns or 1.55 microns). Parametricamplifiers are capable of reproducing even femtosecond pulses. Otherdevices may be used to perform this frequency conversion as mentionedearlier.

Using the fiber optic arrangement (31, 32, 33, 34, 35, 36) tocommunicate signals to be multiplexed reduces heat loss as glass doesnot conduct as much heat into the dewar as copper electrical wireswould. In addition, the fiber optic arrangement has better bandwidth,lower crosstalk, and avoids ground-level feed through. An alternativeembodiment that eliminates the optical fibers 33 and 34 is alsoadvantageous. A free space optical communication link is establishedthrough the transparent dewar (or a transparent window of the dewar).This eliminates heat loss because there is no physical link to carryheat into the dewar. A vertical cavity surface emitting laser (VCSEL)array and charged coupled device (CCD) array would be especiallydesirable in this type of arrangement.

Returning to FIGS. 3A-3H and 4A-4H the "H" configuration of thesuperconducting layer 50 provides several advantages. The two legs 50aof the H shape allow for low resistance electrical contact with theshift register 16'. The narrower bridge 50b part of the H allows thispart of the superconducting layer to switch faster. The switching speedis linearly related to the width of the bridge as shown by equation {4}.In addition, the narrower bridge reduces the amount of current requiredto switch the switch to its partially transparent non-superconductingstate. This reduces the dissipative heating in the switch.

A more detailed explanation of the current flow through the modulatingdevice 14' is given below. The following dimensions concern thesuperconducting layer 50.

W1 is the width of the first contact segment 50a.

L1 is the length of the first contact segment 50a.

T1 is the thickness of the superconducting thin film in the firstcontact segment 50a.

W2 is the width of the light impinging segment 50b.

L2 is the length of the light impinging segment 50b.

T2 is the thickness of the superconducting thin film in the lightimpinging segment 50b.

W3 is the width of the second contact segment 50a.

L3 is the length of the second contact segment 50a.

T3 is the thickness of the superconducting thin film in the secondcontact segment 50a.

A good conductor (such as gold) is deposited on the surface of the firstand second contact segments 50a. The layer of gold should at leastpartially cover the surface area of the superconductor defined by(W1×L1) for the first contact segment and (W3×L3) for the second contactsegment. This provides a low resistance contact to the superconductinglayer.

The critical current density J is defined by the electrical current lflowing through a cross sectional area (W1×L1) A Initially the currentflows substantially vertically through the area defined by thegold-superconductor contact. Then the current travels substantially in ahorizontal direction through a cross section of area (W1×T1). Thecurrent then enters the light impinging segment 50b. In at least oneembodiment the width w2 of the cross sectional area of the lightimpinging segment 50b is substantially smaller than the width w1 of thecontact segment 50a and the thickness of the films are the same (T1=T2).Thus the cross sectional area A in the light impinging segment 50b(W2×T2) is smaller and since l is conserved J increases. This increasein J is due to the restriction of the cross sectional area and causesthe light impinging segment to enter its non-superconducting state at alower electrical current than the contact segments. The electricalcurrent then moves into the second contact segment travelingsubstantially in a horizontal direction through a cross sectional area(W3×T3). The current then moves substantially in a vertical directioninto the gold through the area where the gold and superconductor are incontact. This area at least partially covers (W3×L3).

In addition, the present system is compatible with wave divisionmultiplexing (WDM) and soliton transmission.

Another alternative embodiment of the present invention uses differentshapes in the light impinging section of layer 50 to allow for discreteregions to switch. An example of this is shown in FIG. 5 whichillustrates a modified H shaped configuration 50'. Here the legs of theH shape 50a' serve as contact areas in the same manner as legs 50a. Thebridge 50b' is divided into discrete sections 1, 2 and 3, as shown. Thisallows amplitude shift keying (ASK). ASK allows a single pulse to carrymultiple bits of information. By way of illustration and not limitation,when the electrical current passing through the superconducting layer islow, the light output is zero and this can be used to represent the bitstring "00". Notice that section 1 has the most restricted width andtherefore constricts the electrical current to a smaller cross sectionalarea increasing the critical current density in this region for a givenamount of electrical current. The current can then be raised to a leveljust high enough to cause section 1 (but not the other sections) toenter its non-superconducting state, allowing only the light impingingsection 1 to pass light. This small amount of light could be used torepresent the bit string "01". An even higher current could causesection 2 and section 1 to enter its non-superconducting state. Thenonly light impinging section 1 and 2 would pass light. This greateramount of light could be used to represent the binary string "10".Finally an even greater current could be used to cause section 3 toenter its non-superconducting state, preferably this current is not highenough to cause section 50a' (the contact section) to switch. Lightwould then pass through sections 1, 2, and 3 which could be used torepresent the binary string "11". Thus current pulses with differentmagnitude can be used to create light pulses with different magnitude.

Yet another alternative embodiment of the present invention usesdifferent shapes in the light impinging segment to allow for continuousregions to switch, examples of which are shown in FIGS. 6A and 6B. Thisallows analog control of the light amplitude sent. It can be seen fromFIGS. 6A and 6B that the amount of area in section B that is in itsnon-superconducting state increases with an increase in the currentpassing through the superconducting layer. Thus the amount of lightallowed to pass through the superconducting layer is proportional to thecurrent passing through the superconducting layer and can be varied in acontinuous or analog manner.

REFERENCES

1. Collins, R. T. et.al. "Infrared Studies of the Normal andSuperconducting States of YBa₂ Cu₃ O_(7-x)." IBM Journal of RES.&DEV.vol.33, no.3, May 1989, pgs 238-244.

2. Schlesinger, Z et.al. "Infrared Studies of the Superconducting EnergyGap and Normal-State Dynamics of the high-T_(c) Superconductor YBa₂ Cu₃O₇." Physical Review B, vol. 41, no. 16, Jun. 1, 1990, pgs 11237-11259.

3. Tanner, D. B. "Far-Infrared Transmittance and Reflectance Studies ofOriented YBa₂ Cu₃ O₇." Physical Review B, vol. 43, no. 13, May 1, 1991,pgs 10383-10389.

4. Mattis, D. C., Bardeen J. Phys Rev 111, 412 (1958).

5. Lines, M. E., Nassau K. "calculations of scattering loss anddispersion related parameters for ultralow-loss optical fibers." OpticalEngineering vol. 25 no.4, April 1986, Pgs 602-607.

6. Van Zoeghbroeck, B. "Optical Data Communication betweenJosephson-Junction Circuits and Room-Temperature Electronics." IEEETransactions on Applied Superconductivity, Vol.3, No. 1, March 1993, pgs2881-2884.

7. Martens, Jon S. et. al. "High-Temperature superconducting shiftregisters operating at up to 100 Ghz." IEEE Journal of Solid StateCircuits, Vol. 29, No. 1, January 1994, pgs 56-62.

8. Yariv, A. Optical Electronics, 4th edition, chapter 8, HRW press1991.

9. Saleh & Teich, Photonics, chapter 19.

10. Ronson, K. et.al. "Self-Timed Integrated-Optical Serial-to-ParallelConverter for 100 Gbit/s Time Demultiplexing.", IEEE PhotonicsTechnology Letters, Vol. 6 No. 10, October 1994, pgs 1228-1231.

11. Harshavardhan, K. S. "High T_(c) Thin Films Deposited by PLD ontoTechnologically important Substrates." AIP Conference Proceedings 288New York, AIP Press, 1994, pgs 607-612.

12. Kozyrev, A. B. "Fast Current S-N Switching in YBa₂ Cu₃ O_(7-x) Filmsand It's Application to an Amplitude Modulation of Microwave Signal."Sverkhprovodimost April 1993, pgs 655-667.

What is claimed is:
 1. A light modulation system, comprising:(a) a lightmodulating device having an output and providing at its output a trainof light pulses, each of which has a certain wavelength, said lightmodulating device including(i) a source of light having said certainwavelength, (ii) a layer of superconductive material through which lightfrom said source must pass before it can reach the device's output, saidsuperconductive material being switchable between a superconductingstate in which said light can not pass and a non-superconducting statein which said light can pass, and (iii) an arrangement for switchingsaid superconducting material between its superconducting andnon-superconducting states in a way which provides said light pulses atthe output of the device; and (b) a wavelength changing device opticallycoupled to the output of said light modulating device for changing thewavelength of said pulses.
 2. A light modulation system according toclaim 1 including a fiber optic link optically coupled to saidwavelength changing device for receiving said wavelength changed lightpulses and directing them away from the wavelength changing device,wherein the minimum wavelength of the light from said source is limitedby the ability of said superconductive material to act on the light in away that produces said pulses as a result of the material being switchedbetween its superconductive and non-superconductive states and whereinthe change in wavelength caused by said wavelength changing device isdependent on the ability of said fiber optic link to efficiently carrysaid pulses without significant attenuation or dispersion.
 3. A lightmodulation system according to claim 2 wherein the wavelength of saidlight source is no less than approximately 2 microns and wherein saidwavelength changing device reduces the wavelength of said pulses tobetween approximately 0.5 and 2 microns.
 4. A light modulation systemaccording to claim 1 wherein said arrangement for switching saidsuperconductive material between its superconducting andnon-superconducting states includes means for providing input pulses oflight, at least one detector for detecting said input pulses and meansfor switching said superconductive material between its superconductingand non-superconducting states in response to and depending upon thedetected input pulses.
 5. A light modulation system according to claim 4including a fiber optic link optically coupled to said wavelengthchanging device for receiving said wavelength changed light pulses anddirecting them away from the wavelength changing device.
 6. A lightmodulation system according to claim 5 wherein the minimum wavelength ofsaid light source is no less than approximately 2 microns and whereinsaid wavelength changing device reduces the wavelength of said pulses tobetween approximately 0.5 and 2 microns.
 7. A light modulation systemaccording to claim 1 wherein said arrangement for switching saidsuperconducting material between its superconducting andnon-superconducting states includes means for exposing magnetic fieldsto said superconducting material in predetermined ways so as to switchsaid material between said states to provide said light pulses.
 8. Alight modulation system according to claim 1 wherein said arrangementfor switching said superconducting material between its superconductingand non-superconducting states includes means for subjecting saidsuperconducting material to temperature variations in predetermined waysso as to switch said material between said states to provide said lightpulses.
 9. A light modulation system according to claim 1 wherein saidarrangement for switching said superconducting material between itssuperconducting and non-superconducting states includes means fordirecting an electrical current through said superconducting material inpredetermined ways so as to switch at least a portion of said materialbetween said states to provide said light pulses.
 10. A light modulationsystem according to claim 9 wherein said superconductive materialincludes first and second contact segments and a third light impingingsegment, all of which are electrically connected together, wherein saidlight modulating device further includes means for preventing light fromsaid source from reaching the output of said light modulating device bypassing through said contact segments of said superconductive materialirrespective of its superconducting or non-superconducting states whileallowing said material to pass light through its light impinging segmentand reach said output when the material is in its non-superconductingstate, and wherein said means for directing current through saidmaterial includes an electrical circuit including said contact and lightimpinging segments of superconductive material such that the electricalcurrent first passes through one of said contact segments, then throughsaid light impinging segment and thereafter through the other contactsegment.
 11. A light modulation system according to claim 10 whereinsaid light preventing means includes means located either in front of orbehind said first and second contact segments for phyically preventingsaid light from said source from reaching the output of said lightmodulating device.
 12. A light modulation system according to claim 9wherein said superconductive material includes first and second contactsegments and a third light impinging segment, all of which areelectrically connected together, wherein said means for directingcurrent through said material includes an electrical circuit includingsaid contact and light impinging segments of superconductive materialsuch that said electrical current first passes through one of saidcontact segments, then through said light impinging segment andthereafter through the other contact segment in a predetermined way, andwherein said first and second contact segments and said third lightimpinging segments are configured such that when a current of apredetermined magnitude passes through said first, second and thirdsegments in said predetermined way when the latter are in theirrespective superconducting states, only said third light impingingsegment switches to its non-superconducting state in response thereto,whereby light from said source reaches the output of said lightmodulating device though said third segment only.
 13. A light modulationsystem according to claim 12 wherein the surface area defined by each ofsaid contact segments of said superconductive material is larger thanthe surface area defined by said light impinging segment.
 14. A lightmodulation system according to claim 10 wherein said contact and lightimpinging segments together define an H-shaped configuration where saidlight impinging segment serves as the cross-bar of the H-shape.
 15. Alight modulation system according to claim 14 wherein said electricalcircuit includes first and second electrical contacts respectivelyconnected to said first and second contact segments, each of saidcontacts extending parallel to and the length of but being thinner thanits respective contact segment.
 16. A light modulation system accordingto claim 14 wherein said device includes a light opaque support platehaving one surface thereof for supporting said H-shaped superconductivematerial and at least in part serving as said light preventing means,said support plate including means for allowing light passing throughthe light impinging segment of said superconductive material to passthrough the plate also and reach the output of said light modulatingdevice.
 17. A light modulation system according to claim 9 wherein saidsuperconductive material includes first and second contact segments anda third light impinging segment, all of which are electrically connectedtogether, wherein said means for directing current through said materialincludes an electrical circuit including said contact and lightimpinging segments of superconductive material such that said electricalcurrent first passes through one of said contact segments, then throughsaid light impinging segment and thereafter through the other contactsegment in a predetermined way, and wherein said first and secondcontact segments and said third light impinging segments are configuredsuch that when a current of a different magnitudes pass through saidfirst, second and third segments in said predetermined way when thelatter are in their respective superconducting states, certain differentportions of at least said third light impinging segment respectivelyswitch to their non-superconducting state depending upon the magnitudeof said current, whereby light from said source reaches the output ofsaid light modulating device through the switched portion or portions ofsaid third segment.
 18. A light modulation system according to claim 17wherein said different current magnitudes include a first currentmagnitude and a second higher current magnitude and wherein saiddifferent portions of said third segment include a first portion and asecond larger portion which includes said first portion.
 19. A lightmodulation system according to claim 17 wherein said different currentmagnitudes include a first current magnitude and a second higher currentmagnitude and wherein said different portions of said third segmentinclude a first portion and a second distinctly different portion.
 20. Adata transmission system, comprising:(a) a light modulating devicehaving an output and providing at its output a train of light pulsescontaining data, each pulse of which has a wavelength of at least about2 microns, said light modulating device including(i) a source of lighthaving said wavelength of at least about 2 microns, (ii) a layer ofsuperconductive material through which light from said source must passbefore it can reach the device's output, said superconductive materialbeing switchable between a superconducting state in which said light cannot pass and a non-superconducting state in which said light can pass,and (iii) an arrangement for switching said superconducting materialbetween its superconducting and non-superconducting states in a waywhich provides said data containing light pulses at the output of thedevice; (b) a wavelength changing device optically coupled to the outputof said light modulating device for reducing the wavelength of saidpulses to a wavelength between approximately 0.5 and 2 microns; and (c)a fiber optic link optically coupled to said wavelength changing devicefor receiving said wavelength changed light pulses and directing themaway from the wavelength changing device for further transmission, saidoptical link being constructed of a material which efficiently directslight therethrough when the light has a wavelength between about 0.5 and2 microns.
 21. A light modulation system according to claim 20 whereinsaid arrangement for switching said superconductive material between itssuperconducting and non-superconducting states includes means forproviding input pulses of light, at least one detector for detectingsaid input pulses and means for switching said superconductive materialbetween its superconducting and non-superconducting states in responseto and depending upon the detected input pulses.
 22. A light modulationsystem according to claim 21 wherein said means for switching saidsuperconducting material between its superconducting andnon-superconducting states includes means for directing an electricalcurrent through said superconducting material in predetermined ways soas to switch a portion of said material between said states to providesaid light pulses.
 23. A light modulation system according to claim 22wherein said superconductive material includes first and second contactsegments and a third light impinging segment, all of which areelectrically connected together, wherein said means for directingcurrent through said material includes an electrical circuit includingsaid contact and light impinging segments of superconductive materialsuch that said electrical current first passes through one of saidcontact segments, then through said light impinging segment andthereafter through the other contact segment in a predetermined way, andwherein said first and second contact segments and said third lightimpinging segments are configured such that when a current of apredetermined magnitude passes through said first, second and thirdsegments in said predetermined way when the latter are in theirrespective superconducting states, only said third light impingingsegment switches to its non-superconducting state in response thereto,whereby light from said source reaches the output of said lightmodulating device through said third segment only.
 24. A lightmodulation system according to claim 23 wherein the surface area definedby each of said contact segments of said superconductive material islarger than the surface area defined by said light impinging segment.25. A light modulation system according to claim 24 wherein said contactand light impinging segments together define an H-shaped configurationwhere said light impinging segment serves as the cross-bar of theH-shape.
 26. A light modulating means according to claim 25 wherein saidelectrical circuit includes first and second electrical contactsrespectively connected to said first and second contact segments, eachof said contacts extending parallel to and the length of but beingthinner than its respective contact segment.
 27. A light modulationsystem according to claim 25 wherein said device includes a light opaquesupport plate having one surface thereof for supporting said H-shapedsuperconductive material and at least in part serving as said means forpreventing light from said source from passing through said contactsegments of said superconductive material irrespective of its states,said support plate including means for allowing light passing throughthe light impinging segment of said superconductive material to passthrough the plate also.
 28. A light modulation system according to claim22 including means defining a dewar around said superconductive materialsuch that the superconductive material is thermally isolated from theambient surroundings and from said means for providing said input pulsesof light.
 29. A superconductive plate assembly for use in a lightmodulation system which includes a light modulating device having anoutput and providing at its output a train of light pulses, each ofwhich has a certain wavelength, said light modulating device including(i) a source of light having said certain wavelength,(ii) a layer ofsuperconductive material through which light from said source must passbefore it can reach the device's output, said superconductive materialbeing switchable between a superconducting state in which said light cannot pass and a non-superconducting state in which said light can pass,and (iii) an arrangement for switching said superconducting materialbetween its superconducting and non-superconducting states in a waywhich provides said light pulses at the output of the device, said plateassembly comprising: (a) a support member including at least one sectionthereof through which said light pulses can pass; (b) said layer ofsuperconductive material, the latter being supported by said supportmember, said superconductive material including first and second contactsegments for receiving electrical contacts and a third light impingingsegment, all of which are electrically connected together; and (c) meansfor preventing light from said source from reaching the output of saidlight modulating device by passing through said contact segments of saidsuperconductive material irrespective of its superconducting ornon-superconducting states while allowing said material to pass lightthrough its light impinging segment and said one section of said supportmember and reach said output when the material is in itsnon-superconducting state.
 30. A superconductive plate assemblyaccording to claim 29 wherein the surface area defined by each of saidcontact segments of said superconductive material is larger than thesurface area defined by said light impinging segment.
 31. Asuperconductive plate assembly according to claim 30 wherein saidcontact and light impinging segments together define an H-shapedconfiguration where said light impinging segment serves as the cross-barof the H-shape.
 32. A superconductive plate assembly according to claim31 wherein said support plate is a light transmissive support platehaving one surface thereof for supporting said H-shaped superconductivematerial and also supporting said light preventing means.
 33. Asuperconductive plate assembly according to claim 31 wherein saidsupport plate is a light opaque support plate having one surface thereoffor supporting said H-shaped superconductive material and at least inpart serving as said light preventing means, said support plateincluding said one section thereof for allowing light passing throughthe light impinging segment of said superconductive material to passthrough the plate also and reach the output of said light modulatingdevice.
 34. A method of transmitting data comprising the steps of:(a)providing a light modulating device having an output and generating atits output a train of light pulses which is of a given wavelength andwhich contain data; (b) changing the wavelength of said train of lightpulses; and (c ) directing said wavelength changed pulses into a fiberoptic link for transmission away from said light modulatingdevice;wherein said light modulating device includes: (d) a source oflight having a given wavelength, (e) a layer of superconducting materialthrough which light from said source must pass before it can reach thedevice's output, said superconductive material being switchable betweena superconducting state in which said light can not pass and anon-superconducting state in which said light can pass, and (f) anarrangement for switching said superconducting material between itssuperconducting and non-superconducting states in a way which providessaid light pulses at the output of the device.
 35. A method according toclaim 34 wherein the wavelength of the light pulses at the output ofsaid light modulating device is approximately 2 microns and wherein saidwavelength changing device reduces the wavelength of said pulses tobetween approximately 0.5 and 2 microns.
 36. A superconductive plateassembly especially suitable for use in a light modulation system whichincludes a light modulating device having an output and providing at itsoutput a train of light pulses, each of which has a certain wavelength,said light modulating device including (i) a source of light having saidcertain wavelength, (ii) a layer of superconductive material throughwhich light from said source must pass before it can reach the device'soutput, said superconductive material being switchable between asuperconducting state in which said light can not pass and anon-superconducting state in which said light can pass, and (iii) anarrangement for switching said superconducting material between itssuperconducting and non-superconducting states in a way which providessaid light pulses at the output of the device, said switching meansincluding means for directing an electrical current through saidsuperconducting material in a predetermined way so as to switch at leasta portion of said material between said states to provide said lightpulses, said plate assembly comprising:(a) a support member including atleast one section thereof through which said light pulses can pass; and(b) said layer of superconductive material, the latter being supportedby said support member, said superconductive material being configuredsuch that when a current of a predetermined magnitude passes through itin said predetermined way when the latter is in its superconductingstate, only a predetermined portion thereof switches to itsnon-superconducting state in response thereto, said predeterminedportion being in optical alignment with said one section of said supportmember, whereby light from said source is able to reach the output ofsaid light modulating device though said portion and one section only.37. A superconductive plate assembly especially suitable for use in alight modulation system which includes a light modulating device havingan output and providing at its output a train of light pulses, each ofwhich has a certain wavelength, said light modulating device including(i) a source of light having said certain wavelength, (ii) a layer ofsuperconductive material through which light from said source must passbefore it can reach the device's output, said superconductive materialbeing switchable between a superconducting state in which said light cannot pass and a non-superconducting state in which said light can pass,and (iii) an arrangement for switching said superconducting materialbetween its superconducting and non-superconducting states in a waywhich provides said light pulses at the output of the device, saidswitching means including means for directing an electrical current ofdifferent first and second magnitudes through said superconductingmaterial in a predetermined way so as to switch at least differentportions of said material between said states to provide said lightpulses, said plate assembly comprising:(a) a support member including atleast one section thereof through which said light pulses can pass; and(b) said layer of superconductive material, the latter being supportedby said support member, said superconductive material being configuredsuch that when a current of said different magnitudes pass through it insaid predetermined way when the latter is in its superconducting state,first and second portions thereof respectively switch to itsnon-superconducting state in response to said different currentmagnitudes, said first and second portions being in optical alignmentwith said one section of said support member, whereby light from saidsource is able to reach the output of said light modulating devicethough either of said portions and one section only.
 38. A lightmodulation system according to claim 37 wherein said different currentmagnitudes include a first current magnitude and a second higher currentmagnitude and wherein said different portions of said third segmentinclude a first portion and a second larger portion which includes saidfirst portion.
 39. A light modulation system according to claim 37wherein said different current magnitudes include a first currentmagnitude and a second higher current magnitude and wherein saiddifferent portions of said third segment include a first portion and asecond distinctly different portion.